Manipulating the Bulk Band Structure of Artificially Constructed van der Waals Chalcogenide Heterostructures

The bulk band structures of a variety of artificially constructed van der Waals chalcogenide heterostructures IVTe/V₂VI₃ (IV: C, Si, Ge, Sn, Pb; V: As, Sb, Bi; VI: S, Se, Te) have been systematically examined using <i>ab initio</i> simulations based on density functional theory. The crystal structure and the electronic band structure of the heterostructures were found to strongly depend on the choice of elements as well as the presence of van der Waals corrections. Furthermore, it was found that the use of the modified Becke–Johnson local density approximation functional demonstrated that a Dirac cone is formed when tensile stress is applied to a GeTe/Sb₂Te₃ heterostructure, and the band gap can be controlled by tuning the stress. Based on these simulation results, a novel electrical switching device using a chalcogenide heterostructure is proposed

Si-Doping Effects in Cu(In,Ga)Se₂ Thin Films and Applications for Simplified Structure High-Efficiency Solar Cells

We found that elemental Si-doped Cu­(In,Ga)­Se₂ (CIGS) polycrystalline thin films exhibit a distinctive morphology due to the formation of grain boundary layers several tens of nanometers thick. The use of Si-doped CIGS films as the photoabsorber layer in simplified structure buffer-free solar cell devices is found to be effective in enhancing energy conversion efficiency. The grain boundary layers formed in Si-doped CIGS films are expected to play an important role in passivating CIGS grain interfaces and improving carrier transport. The simplified structure solar cells, which nominally consist of only a CIGS photoabsorber layer and a front transparent and a back metal electrode layer, demonstrate practical application level solar cell efficiencies exceeding 15%. To date, the cell efficiencies demonstrated from this type of device have remained relatively low, with values of about 10%. Also, Si-doped CIGS solar cell devices exhibit similar properties to those of CIGS devices fabricated with post deposition alkali halide treatments such as KF or RbF, techniques known to boost CIGS device performance. The results obtained offer a new approach based on a new concept to control grain boundaries in polycrystalline CIGS and other polycrystalline chalcogenide materials for better device performance